Abstract This study investigated the antiviral activities of Radix Isatidis polysaccharide (RIP) against type II herpes simplex virus (HSV-2) in vitro. RIP was prepared from the Radix Isatidis root. The toxicity of RIP on Vero cells was detected. The direct killing effect of RIP on HSV-2, inhibitory effect of RIP on HSV-2 replication and inhibitory effect of RIP on HSV-2 adsorption were determined. Results showed that, RIP in concentration range of 25-800 mg/L had no toxic effect on Vero cells. RIP with different concentrations could not directly inactivate the HSV-2. The effective rates on inhibition of HSV-2 replication and adsorption in 800 mg/L RIP group were 71.57% and 48.37%, respectively, which were the highest among different groups. In conclusion, RIP has the antiviral effect against HSV-2 in vitro. This effect mainly occurs in inhibiting the virus duplication and adsorption.